Sigma receptors are saturable, high affinity binding sites for several important classes of psychotropic drugs, including typical antipsychotic, antidepressant, anticonvulsant, and psychotomimetic compounds. They are likely to contribute to the beneficial and/or side-effect profile of these compounds. Sigma sites are present not only in the central nervous system but also occur in several peripheral tissues, and are expressed in high density in a number of tumor cell lines. Furthermore, they are more highly expressed in rapidly proliferating cells than in quiescent cells. They may thus play an important role in cell function. Our studies have shown that agonists at sigma-2 receptors induce morphological changes, followed by apoptotic cell death. These effects were found to occur in various tumor cell lines, as well as in primary cultures of neuronal cells. Sigma-2 agonists induce both a transient and a sustained rise in intracellular calcium levels in human SK-N-SH neuroblastoma cells, which may play a role in the induction of apoptosis. During the past fiscal year, we continued to characterize the apoptotic effect. Ongoing projects investigated novel and selective compounds which may be agonists or antagonists at sigma-2 receptors. Phenylalkylamines and phenylmorphans are classes of compounds under study. Several compounds have been found which may show promise as sigma-2 receptor antagonists. Such compounds would be invaluable in determining the role of sigma-2 receptors in vivo. Sigma-2 agonists were found to induce apoptosis in various breast tumor cell lines, including MCF-7, MCF-7/Adr-, T47D, and SKBr3. The latter three cell lines harbor mutations in the p53 gene, and are thus rendered resistant to apoptosis induced by some DNA-damaging agents, such as doxorubicin and actinomycin-D. Furthermore, MCF-7/Adr- cells overexpress p-glycoprotein, a drug efflux pump which contributes to drug-resistance. However, the cytotoxic potency of sigma-2 agonists and their ability to induce apoptosis in these cells was not affected by the status of p53, indicating that sigma-2 receptors utilize a mechanism that is independent of this gene. Furthermore, use of a caspase-independent mechanism was indicated by the inability of caspase inhibitors to block apoptosis induced by sigma-2 receptor agonists. Finally, subtoxic doses of sigma-2 receptor agonists were found to markedly potentiate the cytotoxic effect of doxorubicin and actinomycin-D in breast tumor cell lines. This supports the notion that sigma-2 receptors and DNA-damaging agents activate distinct apoptotic pathways, which may synergize when simultaneously activated. These results suggest that sigma-2 receptors are involved in regulation of cell proliferation and may activate a novel apoptotic pathway. It may be possible to exploit this pathway for the development of therapeutic agents which would be active against drug-resistant tumors. Studies aimed at identification of signalling molecules defining this pathway are underway.